The hidden improvements of Paul DeJong

You’ve already heard plenty about Paul DeJong’s burst onto the St. Louis Cardinals scene. You don’t need me to write more about that. If you follow me on Twitter, you’ve probably noticed that I worry about the sustainability of a player with a 26.4% K-BB differential. Put another way, Paul DeJong is striking out almost eight times as much as he’s drawing walks. You don’t want me to write about that either, though, since Ben Markham of Viva El Birdos has written about that twice (most recently here).

Well, today I’m writing about Paul DeJong. However, instead of talking about my concerns with his sustainability, I wonder whether he can grow into a more sustainable profile. After all, he just turned 24 years old on August 2nd. He spent less than two calendar years between his debut in Cardinals rookie ball on June 23rd of 2015 and his Major League debut this season on May 28th.

DeJong is still at an age and point in his career where development is both crucial and realistic. He hasn’t spent enough time at any level to declare that “he is what he is,” but it’s critical that he develops quickly.

Paul DeJong has recorded 190 plate appearances or more at four different levels – 2015 in Peoria, 2016 in Springfield, 2017 in Memphis, and 2017 in St. Louis. At the Major League level, we have plate discipline statistics. We can use those to better judge a player’s approach at the plate. At the Minor League levels, we only have strikeout and walk rates (K% and BB%). Using K% and BB% still focuses more on results than process, but at least we’re down to two metrics over which the hitter exerts a greater degree of control.

Above are DeJong’s full season numbers at each level that he accumulated at least 190 PAs. However, we know DeJong is and has been a work-in-progress. By just looking at full season numbers, we miss whether DeJong improved the more he played at each level.

Instead, we should look at how he adjusted over the course of each stop. To do this, I compiled DeJong’s 15-game rolling K% and BB% at each stint, shown below:

There’s a lot to see here. DeJong’s 15-game K% at each level is in red, and his 15-game unintentional walk rate (uBB%) is in blue. Each line begins with his 15th game at each level, and I excluded DeJong’s two-game return trip to Memphis this past June in the 2017 AAA group.

First off, I don’t think there’s much to see in DeJong’s walk rates. However, DeJong showed consistent improvement this year in Memphis, raising his 15-game uBB% from exactly 0% to 7.8%. We don’t want to make too much out of movements over 190 plate appearances, but maybe DeJong was making better adjustments this year.

Looking at his strike rates, however, reveals a more consistent and positive trend. Each year, he consistently improved his 15-game strikeout rate over the course of the season. In Peoria, he improved from 23% to 15%. In Springfield, he saw a huge strikeout spike at the start of the year and a huge dropoff at the end of the year. Using a more normalized rate, he improved from about 30% at the beginning of the year to about 20% by the end. He experienced another jump to a 31 percent 15-game K% at the start of AAA before working it down to about 23% by the time of his callup.

Upon moving to the MLB, Paul DeJong has continued to improve his rolling K%.

After striking out in about a third of his plate appearances to start his MLB career, he worked his K% down to 20%. Pitchers adjusted, and then DeJong looks like he adjusted again. Overall, he’s shown moderate improvement in his strikeout rate over the course of this MLB stint. He’s also shown (very) modest improvement in his uBB%. It’s still not a great non-contact profile. It’s still a very poor non-contact profile. But it’s a little better than his overall season numbers indicate, and it’s a little better than when he first broke onto the scene.

Additionally, now that Paul DeJong is in the MLB, his plate discipline statistics are publicly available. As I mentioned above, strikeout and walk rates are still results-focused; using plate discipline metrics, we can get more into process. A good process is more likely to sustain good results, and a bad process to lead to bad results. An improving process would support improving results or give us better information.

For whatever reason, FanGraph’s calculates 15-game averages before players accumulate fifteen games played. I inserted the black vertical line at game fifteen, which we’ll use as our starting point just like we have in each of the previous charts.

Here, we can see that an improving plate discipline process is the foundation of DeJong’s improving non-contact profile. His chase rate (O-Swing%), zone-swing rate (Z-Swing%), and contact rate have all trended in a positive direction throughout the year. Paul DeJong is laying off balls out of the zone more often, swinging at pitches in the zone more often, and making more contact with his swings.

This offseason, I developed an xStats (expected stats) model to evaluate player performance. In that model, I found that plate discipline metrics are highly correlated and predictive of strikeout and walk rates (duh). Using xStats and DeJong’s most recent 15-game numbers, I calculated an xK% of 18% and an xBB% of 7%. While DeJong has a long way to go before he reaches those non-contact numbers, the data supports the notion that he is improving at the plate.

Paul DeJong may never have even an average non-contact profile. With his batted ball ability and potential to play the middle-infield positions, maybe he doesn’t need to be. He just needs his non-contact ability to be good enough so that pitchers can’t routinely exploit his holes and sustainable enough to keep him on the MLB roster.

There’s no denying that DeJong’s aggregate strikeout and walk rates paint a troubling picture. However, there is substantial evidence that he is improving even at the Major League level. He owns an encouraging history of improving his non-contact profile throughout the year at every level he’s played.

Paul DeJong is not a finished product. He may never be a finished product. Pitchers might find and abuse the holes in his swing. But perhaps DeJong will continue to improve and adapt as a professional hitter. At the start, his success looked like just a flash in the pan. Lately, his breakout is looking like it has a stronger foundation.